SSD or Hard Drive for Time Machine, and Which Drives a Mac Runs at Full Speed

What Time Machine does, and how often it writes

A plain hard drive is fine for Time Machine, and the reason is in how it works. Time Machine is the backup tool built into macOS, and it runs itself. It keeps hourly backups for the past 24 hours, daily ones for the past month, and weekly ones for every month before that. When the disk is full, the oldest are deleted.

The first backup copies everything, and Apple says it might take longer than you expect. After that, it copies only the files that have changed, so the hourly runs are small.

It also keeps hourly local snapshots on the Mac itself for 24 hours. A snapshot is a record of how your files looked at that moment. And the schedule picks up again when the drive is plugged back in. So a MacBook that meets its drive once a day is still backed up.

Why a hard drive is fine for it

Nothing in that pattern needs speed. Each run copies an hour of changes, in the background. That's why the drive with the most owner ratings on our portable storage list is mechanical: the WD Elements. It's an ordinary hard drive, with a spinning disk inside. For a backup, capacity for the money matters more than speed. And for the same money, a mechanical drive gives you roughly twice the capacity of an SSD.

There's one habit to respect. A mechanical drive must sit still while it works. A knock while the disk is spinning mid-write is the most common way to kill one. And a Time Machine drive is written to every hour it's attached. So it belongs on the desk, not in the laptop bag.

Where an SSD helps

An SSD stores everything on chips, with no moving parts. It helps in three places.

The first backup and a full restore. These are the two jobs that move everything at once, and the only times you wait on the drive.
Being knocked. With no moving parts, it survives being carried while it's writing. If the drive travels with the MacBook, buy an SSD for that reason alone.
Silence and size. There's nothing to hear when it wakes every hour, and the Samsung T7 is about the size of a credit card.

What an SSD doesn't improve is the hourly backups. They copy only what changed, and a hard drive keeps up.

How big the drive should be

Aim for at least twice your Mac's storage capacity, which is Apple's guidance. For a 1TB Mac, that's ideally 2TB.

Time Machine keeps its hourly, daily and weekly versions while there's room, and deletes the oldest when the disk fills. So a smaller disk simply keeps fewer months of history. The Mac tells you when it needs more space.

On our list, that favours the mechanical drive for a 1TB Mac. The WD Elements is a 2TB listing. The five SSDs are 1TB, which is enough for a 512GB Mac and not beyond.

APFS versus exFAT, and why the drive ends up APFS

A drive's format is the way it organises files, and Time Machine cares which one it gets. Apple's preferred format for a Time Machine disk is APFS or APFS Encrypted. Apple's guide to Disk Utility, the Mac's built-in app for drives, describes APFS as the file system used by macOS 10.13 or later.

The same guide files exFAT, with MS-DOS (FAT), under formats compatible with Windows. Time Machine can't back up to a disk formatted for Windows. Many portable drives arrive formatted that way. So when you pick one as the backup disk, macOS offers to erase and reformat it.

The erase is total. Do it before there's anything on the drive to lose.

The port question: USB 3 at 10Gb/s versus Thunderbolt

The latest MacBook Air when this was written has two Thunderbolt 4 (USB-C) ports. Apple lists them at up to 40Gb/s for both Thunderbolt 4 and USB 4. The SSDs on our list are USB 3 drives, not Thunderbolt or USB 4, so they get the port's USB 3 mode. Apple's sheet for the M2 MacBook Air spells that out as USB 3.1 Gen 2, at up to 10Gb/s.

Samsung's T7 page calls that same 10Gb/s link USB 3.2 Gen 2. The names are confusing, but it's the same link. So the T7, and the SanDisk Extreme and Kingston XS1000 with their stated 1,050MB/s read, sit within what the port offers.

The Samsung T9 is rated at up to 2,000MB/s over USB 3.2 Gen 2x2, which is a 20Gb/s mode. To get there, Samsung's footnote says, the host (the computer) and the cable must both support Gen 2x2. That mode is on neither Apple sheet. They list 40Gb/s for Thunderbolt 4 and USB 4, and 10Gb/s for USB 3.1 Gen 2, with nothing in between.

So on a MacBook Air, the T9 drops to the 10Gb/s link, which Samsung says it's backward compatible with. There it runs at much the same speed as the T7. Ten gigabits a second is 1,250 megabytes a second, and that's before overhead (the part of the link the connection uses for itself). So no drive labelled 2,000MB/s gets past that on a Mac.

One drive for backup and files, or two

Apple's answer is two. A drive can be split into volumes, which the Mac treats as separate disks. The Time Machine volume is reserved for backups, and Apple says not to store other files on it. Disk Utility can add a second APFS volume to the same disk. But the two share the space, and the twice-the-Mac guidance gets harder to meet.

There's a plainer reason, too. One drive holding your only copy and your backup is one failure away from losing both. The rule our storage page uses is three copies, on two kinds of media, with one kept elsewhere.

Encryption

A backup disk holds a copy of everything, so losing one matters. Time Machine handles this itself, by encrypting the backup so it can't be read without a password. Turn on Encrypt Backup when you set up the disk, and choose a password. You'll need it to restore, so keep it in a password manager.

That works on any drive, which matters for the Samsung T7 Shield below. Its hardware encryption does the same job, so on a Mac you're choosing which of the two passwords to rely on, not whether to encrypt.

Which of our picks fits which Mac owner

A Mac that stays on a desk: the WD Elements 2TB. It's a plain mechanical drive with no software. It has the most owner ratings on our storage page, and its rating is exceptional. It's USB 3.0 with a USB-A cable (the older, rectangular plug). A MacBook Air's data ports are all USB-C, so you'll need a USB-C cable or an adapter.

A MacBook that gets knocked about: the Samsung T7 Shield 1TB. It's the T7 in a rubber housing, with a published IP65 rating, a three-metre drop figure and 256-bit hardware encryption, according to the listing. Both cables come in the box. On a Mac, Time Machine's encryption covers the same ground as the hardware kind, so pay for the Shield for the housing, not the password. A third-party seller sells it, dispatched by Amazon.

A MacBook whose drive travels with it: the Samsung T7 Portable SSD 1TB. It's credit-card sized, in an aluminium case, with nothing inside to knock. It's USB 3.2 Gen 2, which is the 10Gb/s a MacBook Air delivers. It also shares the highest owner rating on the storage page. The trade-off is capacity: 1TB meets Apple's guidance for a 512GB Mac only. Check the listing for the cable.

Outdoors, on site, in the rain: the SanDisk Extreme Portable SSD 1TB. It has a rubberised, IP-rated shell (the rating is for keeping out dust and water) with a carabiner loop. Its stated 1,050MB/s read is within what the port can carry. And it has the most owner ratings of any portable SSD on Amazon UK. It's bulkier than the T7, and its ruggedness is wasted on a desk. Amazon UK labels it the older model. The newer one needs the same Gen 2x2 port as the T9.

An SSD for backups, without paying for ruggedness: the Kingston XS1000 Portable SSD 1TB. It's the plain one, and the listing calls it pocket-sized. It gives up to 1,050MB/s read over USB 3.2 Gen 2, the same class as the T7 and the SanDisk Extreme, so it fits within a MacBook Air's port too. There's no published IP rating, and the listing mentions no hardware encryption. It's sold by Amazon. And it's 1TB, half the capacity of the hard drives here.

A household with a desktop PC that has a Gen 2x2 port: the Samsung T9 Portable SSD 1TB. On that port, it runs at up to 2,000MB/s. It's drop-resistant from three metres and has a five-year warranty, according to the listing. Both USB-C to C and USB-C to A cables come in the box. On a MacBook Air, it runs at much the same speed as the T7, and costs more.

The seventh pick, the SanDisk Ultra Dual Drive, is a pocket flash drive with USB-C and USB-A ends. It's for moving files between devices, not a backup drive.

Can I use an external SSD for Time Machine?

Yes. Apple asks for an external drive on a USB or Thunderbolt port, formatted APFS or APFS Encrypted. It doesn't distinguish between an SSD and a hard drive. An SSD makes the first backup and a full restore faster, and it survives being carried. It doesn't change the hourly backups, though. And for the same money, it holds roughly half the capacity of a mechanical drive.

Does a Time Machine drive need to be twice the size of the Mac?

Not strictly. That's Apple's guidance, ideally: at least twice the Mac's storage capacity, so 2TB for a 1TB Mac. The size decides how much history you keep, not whether backups work, because Time Machine deletes the oldest backups when the disk is full. The minimum is a disk that holds everything on the Mac once.

Why is my portable SSD slower than advertised on a MacBook Air?

Because the advertised figure belongs to a port the Mac doesn't have. Drives labelled 2,000MB/s, like the Samsung T9, need USB 3.2 Gen 2x2. That's a 20Gb/s mode, and it's on no MacBook Air sheet. Apple lists 40Gb/s for Thunderbolt 4 and USB 4 and, on the M2 model, 10Gb/s for USB 3.1 Gen 2.

A USB 3 drive gets that 10Gb/s link, which is 1,250MB/s before overhead. So the T9 settles at roughly the T7's speed. The T7, the SanDisk Extreme and the Kingston XS1000 are Gen 2 drives, so they fit inside it. If one of those is slower still, suspect the cable. Samsung's footnote names it alongside the host.